[storage] JMT: refactor: extract insert_at_child
[storage] JMT: refactor: extract insert_at_child

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core